html-widget-can-go-back

Arguments:
html-widget

Returns true if there is a previous page in the chain of
recently-visited pages of the specified
html-widget, to which you could return by calling
html-widget-go-back.

Implementation note: there appears to be no way to implement
this function on Windows, and it may not be working properly on Linux,
so this function may go away. The function was intended to indicate
whether a Go Back button should currently be enabled.